The inhalation anesthetic agent, halothane, has been shown to protect normal bone marrow stem cells from doses of arabinosylcytosine and vinblastine sufficient to kill over 99% of leukemia marrow cells. No protective action was found for the malignant cells chemotheraphy by reducing harmful side effects to normal cells. A satisfactory model of cell division is the PHA-lymphocyte culture, in which halothane has been found to inhibit DNA synthesis without harming thymidine transport into the cell. Such cultures will be maintained and studies of DNA, RNA and protein synthesis done, using radioisotopic precursors and liquid scintillation counting. Assays of DNA-ase, DNA polymerase and thymidine kinase will be made, and electron microscopic autoradiographic studies of the attachment and pinocytotic uptake of tritiated mitogenic fraction of PHA will be carried out. When these results are integrated into a plausible hypothesis of halothane action, comparable studies will be made in mouse L1210 Leukemia cells to attempt location of differences in response of these cells and normal lymphocytes.